1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a method of fabricating an alignment layer for an LCD device.
2. Discussion of the Related Art
The related art LCD devices use an optical anisotropic property and polarization properties of liquid crystal molecules to display images. The liquid crystal molecules have orientation characteristics of arrangement resulting from their thin and long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field to them. Accordingly, when the electric field is applied to them, polarization properties of light is changed according to the arrangement of the liquid crystal molecules such that the LCD devices display images. At least one alignment layer is formed to determine an initial orientation of the liquid crystal molecules.
FIG. 1 is a schematic perspective view of a related art LCD device, and FIG. 2 is a schematic cross-sectional view of a related art LCD device.
Referring to FIGS. 1 and 2, an LCD device includes an array substrate B1, a color filter substrate B2 and a liquid crystal layer 40 interposed therebetween. In the array substrate B1, a gate line 12, a data line 24, a thin film transistor (TFT) T and a pixel electrode 28 are formed on a first substrate 10. The gate and data lines 12 and 24 cross each other to define the pixel region P, and the TFT T is formed at a crossing portion of the gate and data lines 12 and 24. The pixel electrode 28 in each pixel region P is connected to the TFT T and receives voltages through the TFT T. The TFT T includes a gate electrode 14, a gate insulating layer 16, a semiconductor layer 18 including an active layer 18a and an ohmic contact layer 18b, a source electrode 20 and a drain electrode 22. The gate electrode 14 is connected to the gate line 12, and the semiconductor layer 18 on the gate insulating layer 16 corresponds to the gate electrode 14. The source and drain electrodes 20 and 22 are disposed on the semiconductor layer 18 and spaced apart from each other. The source electrode 20 is connected to the data line 24. A passivation layer 26 exposing a portion of the drain electrode 22 is formed over the TFT T, and the pixel electrode 28 is formed on the passivation layer 26 such that the pixel electrode 28 is connected to the portion of the drain electrode 22. In addition, a first alignment layer 42 of polyimide is formed on an entire surface of the first substrate 10 having the pixel electrode 28.
In the color filter substrate B2, a black matrix 32, a color filter layer 34 and a common electrode 36 are formed on a second substrate 30 facing the first substrate 10. The black matrix 32 is formed on the second substrate 30 and has a lattice shape. The black matrix 32 corresponds to a non-display region of the first substrate 10. The non-display region of the first substrate 10 includes the gate line 12, the data line 24 and the TFT T. The color filter layer 34 includes sub-color filters 34a, 34b and 34c, and each of the sub-color filters 34a, 34b and 34c having one of red (R), green (G), and blue (B) colors corresponds to each pixel region P. Although not shown, a planarization layer is formed on the black matrix 32 and the color filter layer 34. The common electrode 36 is formed over the black matrix 32 and the color filter layer 34. The common electrode 36 generates an electric field with the pixel electrode 28 such that the liquid crystal layer 40 is driven by the electric field. In addition, a second alignment layer 44 is formed on the common electrode 36.
The alignment layers are formed to determine an initial orientation of liquid crystal molecules of the liquid crystal layer. Orientation process, which is divided into a contact type and a non-contact type, is performed to give the alignment layers orientation properties. In the contact type orientation process, a rubbing cloth is used. There is a physical friction between the alignment layer and the rubbing cloth to form a plurality grooves on a surface of the orientation layer. Due to the grooves, the alignment layer has the orientation properties, and the liquid crystal molecules have a pre-determined orientation. On the other hand, in the non-contact type orientation process, an optical reaction is performed onto the alignment layer to give the alignment layer anisotropic properties. The liquid crystal molecules have a pre-determined orientation due to the anisotropic properties.
Unfortunately, because additional processes, such as changing the rubbing cloth, are required in the contact type orientation process, the contact type orientation process makes production costs of the LCD device increasing. Accordingly, the non-contact type orientation process is the subject of significant research and development. Particularly, when multi domains, where the alignment layer is rubbed to have different initial orientations, are required in one pixel region, the non-contact type orientation process is widely used.
In the non-contact type orientation process, the alignment layer is formed from a polyimide resin including an optical functional group. For example, the optical function group includes cyclobutane dianhydride (CBDA).
FIG. 3 shows a structure of a polyimide resin used for forming an alignment layer according to the related art. Generally, polyimide is a polymeric material having an imide ring and synthesized from an aromatic anhydride and diamine. Particularly, photoreaction imide has an optical functional group, such as a cyclobutane dianhydride (CBDA) ring in FIG. 3. When ultraviolet (UV) rays is irradiated, the cyclobutane dianhydride (CBDA) ring is opened such that the photoreaction imide having the optical functional group is resolved into maleimide (MI) and a photo-oxide reactant.
FIG. 4 shows structures of a polyimide resin, which has a cyclobutane dianhydride (CBDA) ring and an oxi-dianiline (ODA) group before and after UV RAY irradiating, and FIG. 5 is a graph showing dichroism and absorbance of a polyimide resin.
Referring to FIGS. 4 and 5, when UV RAY is irradiated onto the polyimide having the cyclobutane dianhydride (CBDA) ring and the oxi-dianiline (ODA) group, the CBDA ring is opened such that maleimide (MI) and the photo-oxide reactant are generated. In FIG. 4, 1376 cm−1, 1240 cm−1 and 1501 cm−1 represent infrared absorption bands. It is possible to measure the infrared absorption band using a fourier transformation infrared (FT-IR) spectroscopy. Maleimide (MI) obtained by UV RAY irradiating has an infrared absorption band of 1397 cm−1. Moreover, as shown in FIG. 5, an infrared absorbance curve 50 and a dichroism curve 52 of maleimide (MI) having the infrared absorption band of 1397 cm−1 are overlapped to each other. The infrared absorbance curve 50 and the dichroism curve 52 of maleimide (MI) are shown in (b) of FIG. 5. It is interpreted that maleimide (MI) has directionless properties and does not affect properties of the alignment layer.
In addition, when polyimide is photodecomposited, not only a main reaction producing maleimide (MI) but also a side reaction producing undesired products is generated. The side reaction reduces a molecular weight of polyimide such that the heat-resisting property of the alignment layer of polyimide is degraded. Accordingly, the LCD device having the related art photo-oriented alignment layer, there are some problems such as after images.